题解 | #求二叉树的层序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    private ArrayList<ArrayList<Integer>> resList=new ArrayList<>();
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        order(root,0);
        return resList;
    }
    //借用i来表示当前结点是第几层，然后把它加入到相应层的list中。
    public void order(TreeNode node,int i){
        if(node==null){
            return;
        }
        if(resList.size()<=i){
            resList.add(new ArrayList<Integer>());
        }
        resList.get(i).add(node.val);
        //递归
        order(node.left,i+1);
        order(node.right,i+1);
    }
}